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Post subject: Hot Rod Deville III - 3 ecc83's/12ax7's
Posted: Wed Nov 02, 2011 6:19 am
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Hello,

Wondering if someone could explain to me why there are 3 12ax7's in the HRD, or maybe just why there are two together and one apart. The reason for this question is just simply for the reason of tube replacement. Could I or should I replace the lone tube with a balanced tube, or simply keep the 3 JJ's the way they are.

I also plan to replace the 6l6GC's with 6v6 tubes. Any problems with this? Eurotubes claim they produce a really nice sound. And believe me, I'm not starving for power with this amp.

Thanks for any advice.

Shucky


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Post subject: Re: Hot Rod Deville III - 3 ecc83's/12ax7's
Posted: Wed Nov 02, 2011 7:25 am
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Hi shucky,

The three 12AX7s are; V1 and V2 (the two together on the right) are preamp, and V3 is phase inverter (PI). V1 and V2 will be for tone shaping, while V3 is the driver for the output tubes. V1 will have the most impact on tone for both channels, and half of V2 is used for both channels and the other half is only added for the drive channel, so V2 will affect both channels to a lesser degree than V1. While V3 has little-to-no affect on tone, a lower gain tube in this position can increase the amount of clean headroom. It is generally desirable to have a balanced tube in V3 with a matched set of output tubes in V4 and V5. If you got the tubes in a set from Eurotubes, the PI is likely balanced already.

I am also going to try a set of EH 6V6 in my HRDlx (should be here Friday). Just be sure that the tubes that you choose can handle the higher Plate voltage of the HRDvl (485 volts). JJ and GT can both handle the higher voltage, and there are probably other brands that will work too. Also make sure to set the bias properly for the 6V6 tubes, as the current needs to be much lower than for a 6L6. Of course using the 6V6 in your HRDvl will drop the power to around 22 watts or so. :idea:

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Post subject: Re: Hot Rod Deville III - 3 ecc83's/12ax7's
Posted: Wed Nov 02, 2011 10:07 am
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Very informative, thanks.

So let me ask one more question then. What if v1 and v2 are say matched JJ's and v3 is a EH ecc83/12ax7 balanced tube. Does brand make a difference for v3? Also the 6v6's I plan to buy are going to be the new JJ 6v6 s. I am under the impression that these will be ok(plate voltage)wise.

It would seem to that the bias for these 6v6's should be 30 to 40, rather than the 60-90 that 6l6's are rated for. I am currently running my 6l6's at about 75. What is your opinion on what to run the 6v6's at?

**update** When I look at the EH 6v6 data sheet it says not more that 450v(plate voltage)

Thanks again,

Shucky


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Post subject: Re: Hot Rod Deville III - 3 ecc83's/12ax7's
Posted: Wed Nov 02, 2011 10:37 am
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The original HRDvl shows a B+ supply of 485VDC. If the "III" version is similar you may need to reduce that via a supplemental voltage-dividing network to safely run the 6V6 tubes.

Arjay

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Post subject: Re: Hot Rod Deville III - 3 ecc83's/12ax7's
Posted: Wed Nov 02, 2011 12:21 pm
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shucky,

Different brands can have different affect on sound, you can mix and match as you like, no need for all to be the same brand, it's all about sound. Some brands sound better in certain positions than other brands. I have amps with all three tubes the same type and brand, and amps with all three different types and brands. You could have one brand a real clean tube in V1, and another brand that may work better to smooth out the drive channel in V2, and another brand with low gain for the PI. Endless possibilities.

The JJ and GT 6V6 are rated for 500 Plate volts, and Bob at Eurotubes has used the JJs in many HRDvl amps with success, and sells sets specifically for the HRDvl. If the EH tube is rated 450 Plate volts, then I would not exceed the rating, therefore not a good candidate for the HRDvl. I ordered the EH 6V6 for my HRDlx, and the Plate voltage is about 430 volts, as compared to the HRDvl which is 485 +. If you want to use a tube with a lower Plate voltage rating such as the EH, you must do as Retroverbial suggested, lower the amp's Plate voltage. Otherwise, use a tube that is rated for 500 Plate volts in the HRDvl.

Using 6V6 output tubes in the HRDvl, with 485 Plate volts would require an approximate bias reading at the test point in the range of 35 mV to 45 mV. Adjust according to your actual measured Plate volts. :idea:

14 watt tube - 50% idle is (14 x .5)/Plate volts = Plate current for one tube. So 50% at 485 Plate volts would be 14 mA for one tube. Add grid currents (3 mA), then multiply by 2 to get approx 34 mV at the test point.
14 watt tube - 70% idle is (14 x .7)/Plate volts = Plate current for one tube. So 70% at 485 Plate volts would be 20 mA for one tube. Add grid currents (3 mA), then multiply by 2 to get approx 46 mV at the test point.

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Post subject: Re: Hot Rod Deville III - 3 ecc83's/12ax7's
Posted: Wed Nov 02, 2011 1:11 pm
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It sounds like the JJ's and the GT's are the ticket. I would not hazard a pair of EH's without reducing the B+ appropriately.

Arjay

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Post subject: Re: Hot Rod Deville III - 3 ecc83's/12ax7's
Posted: Wed Nov 02, 2011 1:41 pm
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Shucky,

Worth noting that folks do report that the JJ 6V6 is very like a 6L6 and not really representative of the 6V6 tube family.

Notwithstanding comments above re: plate voltage, I'd opt for a GT to at least provide some clear blue water between the different tube sounds.

Would be a bummer to end up with a 22 watt 6L6 amp? :?

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Post subject: Re: Hot Rod Deville III - 3 ecc83's/12ax7's
Posted: Wed Nov 02, 2011 5:19 pm
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Good point Feedbackboy

Might as well mention the "hardness" ratings for output tubes while we're at it. GTs have a number and color code for their tubes, indicating breakup point (distortion). Blue 1 to 3 for early breakup, White 4 to 7 mid-range, and Red 8 to 10 for clean headroom. And I should point out that it is the GT 6V6 S that is rated for the higher Plate volts.

Other brands can also be sorted for rating if you call the store and talk to them about what you want. I know that TheTubeStore does this with a number rating for their tubes, and I think Bob at Eurotubes does also, as well as other sellers.

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